Loudspeakers generally comprise a frame, a motor structure, a diaphragm, a lower suspension or spider and an upper suspension or surround. In one common type of speaker, the motor structure includes a permanent magnet mounted between a top plate and a back plate, a pole piece centrally mounted on the back plate and a voice coil axially movable with respect to the pole piece. The voice coil includes a hollow, cylindrical-shaped former having an outer surface which receives a winding of wire. In some designs, the former is formed with a number of circumferentially-spaced bores at a location spaced from where the wire winding is located.
One end of the diaphragm is connected to the surround which, in turn, is mounted to the upper end of the frame. The spider is connected at one end to a seat formed in the frame at a point between its upper and lower ends. The free ends of the diaphragm and spider are mounted to the voice coil and support it within the magnetic gap formed between the pole piece and top plate of the motor structure, with the former of the voice coil concentrically disposed about the pole piece. In some speaker designs, a dust cap is mounted to the diaphragm in position to overlie the voice coil and pole piece to protect them from contaminants. This forms a dust cap cavity between the dust cap, diaphragm and the voice coil and pole piece. In alternative designs, the upper end of the voice coil is connected directly to the diaphragm, thus eliminating the need for a dust cap but nevertheless forming an internal or dust cap cavity in the area directly above the voice coil and pole piece.
In the course of operation of a speaker of the type described above, electrical energy is supplied to the voice coil causing it to axially move relative to the pole piece and within the air gap formed between the top plate and pole piece. The diaphragm, spider and surround, move with the excursion of the voice coil.
Often a loudspeaker design can be best optimized by utilizing a voice coil with a small diameter. However, the smaller voice coil setup creates certain problems, especially when designing loudspeakers for low frequency reproduction. Thus, for larger diameter loudspeakers (typically 10 inches and above), small voice coil systems are not common. Accordingly, there are few, if any, existing diaphragms tooled for the smaller diameter voice coil formers. To incorporate a small voice coil system, the diaphragm must be customized, adapted or re-tooled.
One disadvantage of mating a diaphragm directly to a smaller voice coil is that a relatively small adhesive joint is made. Since the voice coil's diameter is much smaller, the gluing circumference is drastically reduced. Therefore, the designer must be concerned with the possibility of mechanical failures since the stress distribution around the glue joint is high. Because the spider attaches at this critical junction as well, spider joint stress also increases, introducing yet another possible failure mode.
Another problem associated with smaller voice coils occurs in the use of pole vents. Pole vents comprise holes bored directly through the pole piece within the motor structure. These vents are used to relieve air pressure that builds up within the dust cap cavity. Without a pole vent, audible noise can be introduced as the trapped air tries to escape during large cone excursions. However, when using a small diameter voice coil, the amount of metal in the pole piece is very limited. Such amount of steel can only support a limited amount of magnetic flux. Consequently, using a pole piece with large amounts of metal removed for pole vents can radically alter the performance of the magnetic circuit.
A vented pole piece further affects the thermal behavior of the speaker. The steel contained in the pole piece provides an effective thermal sink for the voice coil. Machining a pole vent in the pole piece increases thermal resistance of the sink, lowering the power handling capability of the loudspeaker.
The mechanical integrity of the spider is also compromised when using a small voice coil. Spiders are typically made from resin treated cloth materials. When the inner diameter of the spider gets smaller, fewer strands of material intersect the cutout. Since the glue joint lies on this small circumference, very little spider material is captured. This places the spider material under greater stress than normal which can cause it to fatigue prematurely.
Several loudspeaker designs are contemplated in the background art for improving speaker performance, stabilizing the speaker diaphragm, and/or simplifying the manufacturing process. However, none of these references solve the above-noted problems. For example, Mitobe (U.S. Pat, No. 5,111,510) discloses a speaker and manufacturing method therefor including a diaphragm integrally combined with a first frame piece and a driver unit integrally combined with a second frame piece. Saiki et al. (U.S. Pat. No. 5,371,805) discloses a speaker and speaker system employing the same, comprising a diaphragm secured to a first periphery of an edge member and a frame secured to a second periphery of the edge member. Scholz (U.S. Pat. No. 5,323,469) discloses a conical loudspeaker having a conical stabilizing element joined between an underside of a speaker membrane and an outside surface of a speaker moving coil carrier. Kreitmeier (U.S. Pat. No. 5,424,496) discloses an electromagnetic converter comprising an internal magnet system, a moving coil and tubular segment. Kreitmeier (U.S. Pat. No. 4,764,968) discloses a disk-like diaphragm made from a conical plastic film and provided with vacuum formed support members which extend up to the disk-like radiating layer. Finally, Kobayashi (U.S. Pat. No. 4,118,605) discloses a coil mount structure comprising a cylindrical member, around one end portion of which a diaphragm edge is fixed, an inner peripheral edge portion where a damper is removably fixed, and an opposite end portion around which a coil is provided. Kobayashi, however, does not provide any structure for ventilating air pressure from beneath the dust cap cavity.